As films having high gas barrier properties for food packaging or the like, a film has been known in which a base film is coated with a vinylidene chloride copolymer (hereinafter, simply referred to as PVDC). However, the burnt PVDC generates a toxic or hazardous gas, and in addition, produces an organic chlorine-containing compound with high carcinogenicity upon incineration at low temperatures. For such reasons, switching over from PVDC to other materials has been strongly desired.
Moreover, as films having high oxygen-gas barrier properties, a film coated with a polyvinyl alcohol (hereinafter, simply referred to as PVA) or an ethylene-vinyl alcohol copolymer (hereinafter, simply referred to as EVOH) has been also known. Due to high hydrophilicity of PVA and EVOH, however, such a film tends to deteriorate gas barrier properties (particularly barrier properties against water) remarkably under a highly humid environment.
Japanese Patent Application Laid-Open No. 93133/1994 (JP-6-93133A, Patent Document 1) discloses a resin composition containing a resin and an inorganic layered compound (expansive clay mineral) having a particle size of not more than 5 μm and an aspect ratio of 50 to 5000, or a film using the composition. This document mentions that examples of the resin include a polyvinyl alcohol, an ethylene-vinyl alcohol copolymer, a polyvinylidene chloride, a polyacrylonitrile, a polysaccharide, a polyacrylic acid and an ester thereof, and that the resin is preferably a resin highly forming a hydrogen bond.
Japanese Patent Application Laid-Open No. 57066/1994 (JP-6-57066A, Patent Document 2) discloses an aqueous composition consisting of an aqueous dispersion of an ethylene-vinyl alcohol-series copolymer which may have an ionic group, and an inorganic filler, wherein the ethylene content of the ethylene-vinyl alcohol-series copolymer is 15 to 65% by mol. This document describes that a water-swelling philosilicate (such as a montmorillonite) is preferred as the inorganic filler.
Japanese Patent Application Laid-Open No. 48994/2001 (JP-2001-48994A, Patent Document 3) discloses a gas barrier film which comprises a thermoplastic resin base (e.g., a polypropylene film), and a coating layer containing an inorganic plate-like particle and a water-soluble polymer (e.g., a polyvinyl alcohol-series polymer) as constituent components, the coating layer being formed on at least one side of the base, wherein the film is defined by the relationship between the thickness of the coating layer and the number of projections having a given height on the surface of the coating layer.
However, since the water-soluble or water-dispersive polymer (particularly a water-soluble polymer which highly forms a hydrogen bond) is used in these compositions, coating agents employing these compositions are high in viscosity, as a result, it is difficult to apply (or coat) these compositions in a high concentration. Further, due to easiness of water retention, the coating layer is low in drying characteristic and remains insufficient in gas barrier properties under a highly humid condition.
Japanese Patent Application Laid-Open No. 98047/2001 (JP-2001-98047A, Patent Document 4) discloses a gas barrier polyurethane resin which is obtained by a reaction between a diisocyanate component and a C2-8 alkylene glycol, wherein the total concentration of the urethane group and the urea group is not less than 15% by weight. This document mentions that an aqueous dispersion is obtained by allowing a dihydroxycarboxylic acid such as dimethylolpropionic acid to react with an alkylene glycol to produce a prepolymer, neutralizing the prepolymer with an amine, and elongating the prepolymer chain with the use of a chain-extension agent such as a diamine or a hydrazine. Further, the document also describes that the polyurethane-series resin may contain an expansive inorganic layered compound or a silane coupling agent. Use of the urethane-series resin ensures to improve gas barrier properties. However, higher gas barrier properties are desired for the aqueous (or water-soluble) urethane-series resin.    [Patent Document 1] JP-6-93133A (Claims)    [Patent Document 2] JP-6-57066A (Claims)    [Patent Document 3] JP-2001-48994A (Claims)    [Patent Document 4] JP-2001-98047A (Claims, Paragraph Numbers [0035] to [0039], and [0076] to [0079])